Add shrinker test for interface implementation being deserialized (#2845)

For R8 this currently fails, see TODOs in the new code.

Author: Marcono1234

test-shrinker/src/main/java/com/example/InterfaceWithImplementation.java

@@ -0,0 +1,21 @@
+package com.example;
+
+import com.google.gson.annotations.SerializedName;
+
+/** Interface whose implementation class is only referenced for deserialization */
+public interface InterfaceWithImplementation {
+  String getValue();
+
+  // Implementation class which is only referenced in `TypeToken`, but nowhere else
+  public static class Implementation implements InterfaceWithImplementation {
+    public Implementation() {}
+
+    @SerializedName("s")
+    public String s;
+
+    @Override
+    public String getValue() {
+      return s;
+    }
+  }
+}

test-shrinker/src/main/java/com/example/Main.java

@@ -53,6 +53,8 @@ public static void runTests(BiConsumer<String, String> outputConsumer) {
     testJsonAdapterAnnotation(outputConsumer);
 
     testGenericClasses(outputConsumer);
+
+    testDeserializingInterfaceImpl(outputConsumer);
   }
 
   private static void testTypeTokenWriteRead(
@@ -290,4 +292,25 @@ private static void testGenericClasses(BiConsumer<String, String> outputConsumer
                     "{\"g\": {\"t\": 1}}", new TypeToken<GenericUsingGenericClass<DummyClass>>() {})
                 .toString());
   }
+
+  private static void testDeserializingInterfaceImpl(BiConsumer<String, String> outputConsumer) {
+    Gson gson = new Gson();
+    TestExecutor.run(
+        outputConsumer,
+        "Read: Interface implementation",
+        () -> {
+          try {
+            // Use the interface type here
+            List<? extends InterfaceWithImplementation> list =
+                gson.fromJson(
+                    "[{\"s\": \"value\"}]",
+                    // This is the only place where the implementation class is referenced
+                    new TypeToken<List<InterfaceWithImplementation.Implementation>>() {});
+            return list.get(0).getValue();
+          } catch (ClassCastException e) {
+            // TODO: R8 causes exception, see https://github.com/google/gson/issues/2658
+            return "ClassCastException";
+          }
+        });
+  }
 }

test-shrinker/src/test/java/com/google/gson/it/ShrinkingIT.java

@@ -19,7 +19,7 @@
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.fail;
-import static org.junit.Assume.assumeTrue;
+import static org.junit.Assume.assumeFalse;
 
 import com.example.UnusedClass;
 import java.lang.reflect.InvocationTargetException;
@@ -61,6 +61,11 @@ public void verifyJarExists() {
     }
   }
 
+  /** Returns whether the test is currently running for ProGuard, instead of R8 */
+  private boolean isTestingProGuard() {
+    return jarToTest.equals(PROGUARD_RESULT_PATH);
+  }
+
   @FunctionalInterface
   interface TestAction {
     void run(Class<?> c) throws Exception;
@@ -216,6 +221,10 @@ public void test() throws Exception {
                 "Read: Using Generic TypeToken",
                 "{g={t=read-1}}",
                 "===",
+                "Read: Interface implementation",
+                // TODO: Currently only works for ProGuard but not R8
+                isTestingProGuard() ? "value" : "ClassCastException",
+                "===",
                 ""));
   }
 
@@ -226,7 +235,7 @@ public void testNoSerializedName_NoArgsConstructor() throws Exception {
         c -> {
           Method m = c.getMethod("runTestNoArgsConstructor");
 
-          if (jarToTest.equals(PROGUARD_RESULT_PATH)) {
+          if (isTestingProGuard()) {
             Object result = m.invoke(null);
             assertThat(result).isEqualTo("value");
           } else {
@@ -251,7 +260,7 @@ public void testNoSerializedName_NoArgsConstructorNoJdkUnsafe() throws Exception
         c -> {
           Method m = c.getMethod("runTestNoJdkUnsafe");
 
-          if (jarToTest.equals(PROGUARD_RESULT_PATH)) {
+          if (isTestingProGuard()) {
             Object result = m.invoke(null);
             assertThat(result).isEqualTo("value");
           } else {
@@ -278,7 +287,7 @@ public void testNoSerializedName_HasArgsConstructor() throws Exception {
         c -> {
           Method m = c.getMethod("runTestHasArgsConstructor");
 
-          if (jarToTest.equals(PROGUARD_RESULT_PATH)) {
+          if (isTestingProGuard()) {
             Object result = m.invoke(null);
             assertThat(result).isEqualTo("value");
           } else {
@@ -300,7 +309,7 @@ public void testNoSerializedName_HasArgsConstructor() throws Exception {
   public void testUnusedClassRemoved() throws Exception {
     // For some reason this test only works for R8 but not for ProGuard; ProGuard keeps the unused
     // class
-    assumeTrue(jarToTest.equals(R8_RESULT_PATH));
+    assumeFalse(isTestingProGuard());
 
     String className = UnusedClass.class.getName();
     ClassNotFoundException e =